Spatial Change and Affecting Factors of Soil Cation Exchange Capacity in Tibet

Based on analytical soil data in Tibet, which include 150 profiles of 17 soil types, the spatial change and affecting factors of soil CEC were discussed by statistical method. The result showed that soil CEC had obvious horizon characterics on the vast pleatu. Soil CEC decreased distinctly from southeast to northweast or from the Alpine meadow type, the Alpine steep type to the Alpine desert type. It was in southeast region in Tibet that ver- tical change of soil CEC was also obvious. Soil CEC of mountain wet-forest type soil increased upward from yellow earth in base band, and reached the highest valuable in Dark-brown earth and Podzolic soil. However, above the forest line, soil CEC decreased again form Dark-felty soil, Felty soil to Frigid soil. The spatial change of soil CEC in Tibet was mainly determined by the difference of the accumulation of organic matter. When organic matter content was low, soil CEC was greatly influenced by the clay content. Meanwhile, meadow-form soil in alpine zone was still influenced by silt content. Soil available potassium content had significant positive correlation with CEC, which indicated that soil CEC had important ability to keep potassium.